1. Field of the Invention
The invention relates to moisture-crosslinkable organopolysiloxane-polyurea block copolymers, to a process for preparing the crosslinkable organopolysiloxane-polyurea block copolymers, and to their use; to crosslinkable compositions comprising the crosslinkable organopolysiloxane-polyurea block copolymers, to the preparation of the crosslinkable compositions, to their vulcanization, and to the use of the vulcanizates.
2. Background Art
Organopolysiloxane-polyurea block copolymers are known and are prepared by copolymerizing aminoalkyl-terminated siloxanes and diisocyanates POLYMER, Vol. 25 (1984), p. 1800 f.).
As a result of the formation of hydrogen bonds between the urea groups, these copolymers can be thermoplastic elastomers. Consequently such copolymers are plastic above the softening point while having elastic properties below it. They can therefore be used, for example, as hot-melt adhesives. A disadvantage for the use of such copolymers as hot-melt adhesives, however, is that the adhesive bonding is reversible as a result of an increase in temperature above the softening point. Moreover, moldings or bonds produced from such copolymers are subject to cold flow, since even below the softening point it is possible for hydrogen bonds to undergo continual scission and reformation, with the possible consequences of deformation and hence failure of the desired adhesive function. The field of use is therefore limited to applications in which no high temperatures and/or forces act on the thermoplastic elastomer.
One solution to the problem is to crosslink the individual polymer chains additionally with covalent bonds; bonds which are not thermally reversible. However, where the thermoplastic elastomers are crosslinked during preparation, for example, by using trifunctional units, it is possible for the processing properties such as melt viscosity to be adversely affected.
Patents EP 0 250 248 B1, EP 0 822 952 A1 and DE 101 13 980 A1 describe organopolysiloxane-polyurea block copolymers provided with moisture-crosslinkable silyl end groups, providing the possibility of crosslinking via these end groups following application in liquid melt form. Particularly in the case of long-chain block copolymers with a low end-group concentration, however, the achievable crosslinking density is low, but on the other hand a high degree of polymerization of the block copolymers is necessary in order for thermoplastic properties to develop.